The present invention relates generally to the material handling industry and more particularly, this invention pertains to the overhead material handling industry using applications involving dual hoists.
Within the overhead material handling industry, applications involving dual hoists can be inefficient, costly to implement and wrought with safety concerns. Before the use of Programmable Logic Controllers (PLCs), dual trolley loads were raised utilizing two separate motor and drive packages. Since the hoists operated independently, the loads often would rise at incongruent speeds, causing an un-even lift and potentially unsafe working conditions.
Until recently, the only remedy for this situation was to use a PLC in conjunction with the motor and drive packages. Two drives would be applied to two separate motors and encoders, giving hook position feedback to a PLC. The PLC would control the drives in order to synchronize the speeds of each hook. Though it accomplished the mission of synchronizing the hook speeds, it also increased the complexity and cost of the operating system.
Current products and techniques tend to be either open loop or require an extra sensor of some sort. Open loop products give a simultaneous run command and expect the two hoists to follow the same command well enough to perform a synchronized lift. Other devices require a load cell or some other tension/torque measurement device to detect loading of individual cables and adjust speed on drives based on load. One final method is to monitor position from each motor in an external device, such as a PLC, and then adjust the speed command to individual drives based on the position feedback from their respective motor and encoder.
Several United States Patents have been issued for alternative technologies. These include U.S. Pat. No. 4,266,175, issued to Braun et al. on May 5, 1981; U.S. Pat. No. 4,665,96, issued to Rosman on May 19, 1987; U.S. Pat. No. 5,210,473, issued to Backstrand on May 11, 1993; U.S. Pat. No. 5,324,007, issued to Freneix on Jun. 28, 1994; U.S. Pat. No. 5,625,262, issued to Lapota on Apr. 29, 1997; U.S. Pat. No. 5,874,813, issued to Bode et al. on Feb. 23, 1999; and U.S. Pat. No. 6,047,581, issued to Everlove, Jr. et al. on Apr. 11, 2000.
U.S. Pat. No. 4,266,175 issued to Braun, et al. on May 5, 1981 discloses a method for thyristor control of AC wound rotor motors. This patent involves controlling the switching devices which generate the variable frequency output voltage to a motor.
U.S. Pat. No. 4,665,696 issued to Rosman on May 19, 1987 discloses a hydraulically operated hoist for containerized freight or the like. As may be noted in the claims section, this patent refers specifically to a lift system that is hydraulically actuated. Additionally, per Column 10, lines 30-39 and FIG. 5, the ability to help level the load is produced through a level-sensitive transducer. This transducer, in turn, causes the hydraulic pressure to adjust the load to be leveled.
U.S. Pat. No. 5,210,473 issued to Backstrand on May 11, 1993 discloses a system with delay timer for motor load equalization. This patent is directed to a circuit utilizing a control circuit providing a motor speed signal. Two separate motor connected inverters monitor the signal and generate command ramps for the motor speed control. Each inverter includes a microprocessor means which repetitively runs through its program to scan a sequence of program instructions. One of the items read is the motor speed signal which is utilized to control the speed of the motor. The essential purpose of this device is to attempt to provide a more uniform reference to both motor drives. These motor drives are run asynchronously with each motor following the commands of their respective drives. By setting internal parameters related to acceleration, deceleration, or other pertinent speed control parameters, a similar path will be followed. This device attempts to allow each motor and drive to proceed through initial start-up conditions, such as receiving a run command, generating initial torque, and opening the brake, and then wait at some speed for a set dwell time to ensure both motors are ready to run at the commanded reference speed. At this point the motors begin to follow the independent command trajectories generated by their respective drives.
U.S. Pat. No. 5,324,007 issued to Freneix on Jun. 28, 1994 discloses a load-hoisting system having two synchronously rotating drums operating in parallel. This system has a single motor and controller driving two output shafts. This patent is for a system that is mechanically redundant in order to prevent a load from falling due to a single mechanical failure.
U.S. Pat. No. 5,579,931 issued to Zuehlke, et al. on Dec. 3, 1996 disclosing a system for a lift crane with synchronous rope operation. This method is used by a lift crane which uses two separate ropes attached to a single hook in such a manner that tension can be measured between the two ropes. If the tension changes such that it indicates one of the ropes is moving faster than the other, the speed can then be adjusted so that the two ropes lift at the same speed.
U.S. Pat. No. 5,625,262 issued to Lapota on Apr. 29, 1997 discloses a system for equalizing the load of a plurality of motors. This patent details a method of load sharing between two drives utilized in tandem to control a single hoist. This is accomplished by issuing a torque reference command from the first inverter to the second inverter as noted in column 3, lines 3-16. In column 3, lines 17-30 of this patent, it is claimed that a speed indication of the first motor is sent to the second motor to assist in controlling the speed of the second motor. The only connection between the two drives that is necessary and/or discussed is line 150 of FIG. 3 as referenced in column 7, lines 31-34. This is the torque reference generated by the first drive, labeled 96, and sent to the second drive, labeled 94. Column 4, lines 59-64, reference controller operating by a lever to provide input signals to the drives producing a speed command for the drives. This is one of two common methods of generating a speed command to a drive. This allows for an analog command signal with a range of speed commands from the minimum programmed speed up to the maximum programmed speed. The second method typically uses pushbuttons, but could be any type of discrete input, to generate discrete speed input commands corresponding to pre-programmed levels. This is common practice in the crane and hoist industry.
U.S. Pat. No. 5,874,813 issued to Bode et al. on Feb. 23, 1999 discloses a control method, especially for load balancing of a plurality of electromotor drives. As noted in the background of this patent it is known in the art to utilize a control process in which the difference between the armature currents of two successive drives produces a signal which is used to reduce the speed setpoint in the speed control circuit of the more strongly loaded drive to bring about a load balancing. As noted in Column 4 each of the electric motors have a separate speed control circuit which comprises a speed controller and a proportional feedback unit connected in parallel to the controller. As noted in Column 5, beginning at Line 4, the output of the speed controller is feed into an adder so that the setpoint value can be corrected and delivered to the current controller. The primary purpose of this controller is to provide the proper torque or tension throughout a system in which material is pulled through or across multiple points by multiple motors. In this type of application, controlling the tension is typically the most desired feature of a control system. This explains the primary concentration on controlling current, as torque is directly proportional to current. As stated in column 3, lines 26-30, the effect of the speed feedback controller is limited to allow the separate load-balancing controller to dominate performance in this system.
U.S. Pat. No. 6,047,581 issued to Everlove Jr., et al. on Apr. 11, 2000 discloses a drive system for vertical rack spline-forming machine. This patent discloses the use of two or more motors for driving a spline-forming machine. This invention utilizes a PLC to provide output to two circuit motor power control modules which advance the slide. As noted by the description in this patent a home position is utilized to synchronize the position of the two motors. In the machine tool industry, it is common practice to synchronize a mechanical component which requires dual (multiple) drives such as these rails on a slide by using some sort of electronic home position and an external controller then to keep the two (or more) servomotors running synchronously.
Current control methods typically utilize one of the following methods for synchronizing multiple hoists:
Mechanical coupling between the hoist drums combined with load sharing between the motor drives.
An external sensor to detect differences in speed, alignment or loading of hooks and use of the information to align the hooks.
An external controller used to receive a speed reference and an encoder feedback from each motor drive and use this information to provide the appropriate reference to each drive to maintain alignment of the hooks.
What is needed then is a simplified construction and system for a Multiple Hoist Synchronization Apparatus and Method.
The hoist synchronization software package allows one or more driven motors to be synchronized to a master encoder signal for driving hoist motors. With the present invention""s apparatus and method, a Programmable Logic Controller (PLC) is no longer necessary. In its place a master and slave inverter operation is used to control the hoists. The master encoder provides a pulse reference to the slave that results in the slave commanding its motor to rotate at the speed commanded by that pulse reference. The slave drive, implemented as a Variable Frequency Drive (VFD), monitors the pulse feedback from both the master encoder and the slave""s own encoder. The slave will then compensate for any position errors by adjusting its motor""s output speed, resulting in near perfect alignment between the system master motor and the slave motor. While both drives are running there is no accumulation of position error, so alignment will always be maintained.
Additionally, when utilizing the new hoist software, the slave VFD possesses the ability to automatically resynchronize the hoists. Automatic resynchronization can be used in multiple configurations. This feature is enabled or disabled on via parameter settings that can provide three optional settings of 0xe2x80x94no automatic resynchronization (hold error), 1xe2x80x94automatic synchronization enabled with position error zeroed by upper limit (synchronize), and 2xe2x80x94automatic synchronization enabled with position error zeroed by multi-function input (synchronize with clear error).
With a parameter setting of 0xe2x80x94no automatic resynchronization (hold error), the slave will hold the position error to zero when either drive operates independently. Thus the resynchronization function is disabled. Once the drives are stopped and a command is given to utilize both hoists together, they will maintain their cur-rent position relative to one another.
With the parameter set to 1xe2x80x94automatic synchronization enabled with position error zeroed by upper limit (synchronize), both hoists can be run to the upper limit and any accumulated position error is cleared out. From that point the hoists will maintain their respective positions to one another. If one hoist is run individually and then both hoists are synchronized again, they will be resynchronized to their initial relative positions to one another without having to go to the upper limits to even them out.
With a setting of 2xe2x80x94automatic synchronization enabled with position error zeroed by multi-function input (synchronize with clear error), the accumulated position error can be cleared at any point by using a multi-function input. This allows the hoists to be set to any position, either aligned or offset from each other, and the accumulated position error is cleared. The hoists will then run together at their respective positions while in the hoist synchronization mode. If one hoist is run individually and then both hoists are run again, they will resynchronize to their respective positions without having to again clear the position error with the multi-function input.
The slave VFD also possesses an electronic gearing feature that allows for synchronization of two or more hoist systems that have unequal hook speeds due to mechanical differences. Consequently the slave can operate at a ratio of the master as though the two were mechanically coupled through belts or gearing.
There are several benefits of utilizing the hoist software, these include: the software allows for independent operation of hoists with resynchronizing capability; the software provides automatic resynchronization between two or more hoists; the software accommodates systems having unequal hook speeds; the software compensates for variations in the encoder PPR between two or more hoists; the software enhances safety by improving control; the software reduces complexity and cost by eliminating the need for a PLC; and the software compensates for mechanical differences between two hoist systems.
The objects and advantages of the invention include: a method of performing synchronization of hoists using encoder feedback from the master motor as a command reference to slave drives; a method of performing functions internal to the drive, some relays are required but no external processor is required; providing the ability to synchronize at any relative position and not just in line with each other; the ability to automatically realign hooks to previous relative position at the beginning of the next run command; and the ability to synchronize non-identical systems. (e.g. different motor speeds, different mechanical gear ratios, or different encoder pulses per revolution).
The present synchronization method is an improvement over the current state of the art in the following ways.
No mechanical coupling is used between any parts of the individual hoists.
The position measurement is obtained from the motor encoders which are already present in the system so no additional sensors or measurements are needed.
All programming is performed in the motor drive, so no additional external controller is required.
The apparatus can easily be configured to synchronize either two or multiple hoists.
Any relative alignment between the hoists can be maintained throughout a lift whereas the typical state of the art typically allows only one relative position (usually in direct alignment) to be maintained.
Different relative alignments between the hoists can be maintained on different lifts in the event that the customer must lift objects of varying size and shape.
The system can automatically restore the last relative alignment between hoists if the individual hoists are run independently and then it is desired that they run synchronously.
The hoists do not need to return to a specific reference point to resynchronize the system.
In addition to these improvements over the prior art, the present hoist synchronization system has the following capabilities:
1. Each hoist is held at zero speed, or a fixed position, until both motors have completed the initial start-up conditions and are ready to run.
2. The present invention performs the hoist synchronization within the motor drives. The slave drive(s) will follow the master drive rather than each drive generating its own command trajectory. This is important because testing has indicated that even if all things are supposedly equal (i.e. motors, drives, parameters, mechanical gearing, etc . . . ) and the motors follow independent trajectories from their respective drives, the motors can end up being one or more revolutions out of position from each other at the end of a commanded run. Effectively this is the difference between an open loop control method used in the prior art, and a closed loop control method used in the present hoist synchronization software.
3. Prior art delay timer circuits must be experimentally adjusted to allow the proper delay time for each system on which it is applied. The present synchronization software has the advantage of reading internal drive signals from both the drive it is installed on as well as the appropriate signals from the other drives to generate a timing independent control system. This control system will simply wait for each drive to reach the appropriate xe2x80x9creadyxe2x80x9d state before continuing operation. As this may vary slightly between individual runs, the synchronization control allows the most efficient starting between multiple drives.
4. Prior art control systems are primarily concerned with controlling the current to achieve desired torque control whereas the present invention is concerned with controlling position between two or more hoists.
5. The system can set a reference point at any position without adjusting an electronic datum point.
These advantages and methods will be explained in the detailed discussion to follow.